An aqueous battery using an electrolyte with a pH of 7 and suitable for direct environmental discard

The electrolytes of aqueous batteries are mostly acidic or alkaline, resulting in inevitable side reactions. More importantly, when batteries are disposed of, strongly acidic and alkaline solutions harm the environment. In this study, we synthesized covalent organic polymers as negative electrode materials for aqueous divalent ion (Mg2+ and Ca2+) storage, which exhibit favorable performance in electrolytes with a pH of 7.0 (the electrolytes are environmentally benign and can even be “brine” in tofu production). Polymers containing electron-donating linking bonds exhibit fast kinetics and low potential for use in aqueous Mg2+/Ca2+ batteries. In neutral electrolytes (pH = 7.0), Hexaketone-tetraaminodibenzo-p-dioxin covalent organic polymers exhibit 120,000 cycles and deliver specific capacities (up to 112.8 mA h g-1). The negative electrode was paired with a positive electrode, creating a full cell that has a voltage interval of 2.2 V and achieves a specific energy of up to 48.3 Wh kg-1, which is calculated on the basis of the total mass of the negative electrode, positive electrode, and electrolyte at the full cell level, together with long-term cycling stability over 120,000 cycles at a specific current of 20 A g-1. The full cells are environmentally benign and nontoxic and can be directly discarded to environments according to various standards (GB 18599-2020, ISO 14001, Resource Conservation and Recovery Act RCRA, US). Aqueous batteries often rely on acidic or alkaline electrolytes that cause side reactions and environmental concerns. Here, the authors develop a covalent organic polymer negative electrode that enables long-life Mg²⁺/Ca²⁺ storage in neutral, environmentally benign electrolytes.